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Ring imaging Cherenkov detector of PHENIX experiment at RHIC
Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
Ring imaging Cherenkov detector of PHENIX experiment at RHIC Y. Akiba *, R. Begay, J. Burward-Hoy, R. Chappell, D. Crook, K. Ebisu, M.S. Emery, J. Ferriera, A.D. Frawley, H. Hamagaki, H. Hara, R.S. Hayano, T.K. Hemmick, M. Hibino, R. Hutter, M. Kennedy, J. Kikuchi, T. Matsumoto, C.G. Moscone, Y. Nagasaka, S. Nishimura, K. Oyama, T. Sakaguchi, S. Salomone, K. Shigaki , Y. Tanaka, J.W. Walker, A.L. Wintenberg, G.R. Young High Energy Accelerator Research Organization, Ibaraki 305-0801, Japan State University of New York - Stony Brook, Stony Brook, NY 11794, USA Florida State University, Tallahassee, FL 32306, USA Nagasaki Institute of Applied Science, Nagasaki 851-0193, Japan Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Center for Nuclear Study, University of Tokyo, Tokyo, 188-0002 Japan University of Tokyo, Tokyo 113-0033, Japan Waseda University, Tokyo 169-8555, Japan
Abstract The RICH detector of the PHENIX experiment at RHIC is currently under construction. Its main function is to identity electron tracks in a very high particle density, about 1000 charged particles per unit rapidity, expected in the most violent collisions at RHIC. The design and construction status of the detector and its expected performance are described. 1999 Elsevier Science B.V. All rights reserved.
The PHENIX experiment [1,6] at the Relativistic Heavy Ion Collider (RHIC) is currently under construction. When completed in 1999, RHIC will provide collisions of Au#Au ions at (s"200 GeV per nucleon. The primary goal of PHENIX is to detect a new state of matter, the quark gluon plasma, in heavy ion collisions. The PHENIX detector consists of 4 spectrometers, or
* Corresponding author. E-mail address: [email protected] (Y. Akiba)
`armsa and an inner detector. Two central arms are to measure charged hardons, photons and electrons at the mid-rapidity. They are placed in back-toback fashion on both sides of the beam line, covering 70}1103 degrees in polar angle (h) and 903 per arm in azimuth ( ). The other two arms are to detect muons in forward and backward angles. The inner detector is to measure the total multiplicity, the vertex position, and the start timing of an event. The PHENIX RICH is the primary device for electron identi"cation in the PHENIX central
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 3 1 9 - 8
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arms. It is a threshold gas Cherenkov detector with a high angular segmentation to cope with the high particle density, about 1000 charged particles per unit rapidity, expected in the most violent collisions at RHIC. Its Cherenkov threshold for pions is 3.5 GeV/c (with ethane radiator) or 4.9 GeV/c (with CO radiator), and thus most pions and heavier particles produce no signal in the RICH. Fig. 1 shows a side view of the RICH detector, and illustrates its function. When an electron passes through the RICH gas volume, Cherenkov photons emitted from the electron are re#ected by thin spherical mirrors and are focused on arrays of PMTs, forming a ring shaped pattern. The direction of the electron track is then measured from the position of the Cherenkov ring. A charged particle track, measured by tracking system of the central arm, is identi"ed as an electron if its direction matches the direction measured from the ring. There are two identical RICH detectors, one for each of the two central arms. The detector is sandwiched by the inner and outer tracking stations of
the PHENIX central arm, and occupies the radial region between r"2.575 and 4.1 m from the beam line. Fig. 2 shows a cut away view of one of the RICH detector. The gas vessel is fabricated from aluminum, and has a volume of approximately 40 m. The vessel houses two arrays of spherical mirrors and two arrays of photo-multiplier tubes (PMTs). The PMT arrays are placed behind the PHENIX central magnet so that particles from the collision vertex do not directly hit the arrays. The vessel is "lled with ethane or CO at 1 atm as Cherenkov radiator. The radiator length, from the entrance window to the surface of the mirror, varies from about 0.9 m at h"903 to 1.5 m at h"703 and 1103. The entrance and exit windows (9 and 21 m) of the vessel are made of 125 lm thick aluminized Kapton. The windows are covered by a light absorber sheet and are supported against a small pressure of radiator gas (1/2 of water) by graphite epoxy rods. All the gas seals are made using GoreTex gaskets.
Fig. 1. A cut through view of PHENIX RICH detector.
Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
145
Fig. 2. Cutaway view of one arm of the PHENIX RICH detector.
In order to minimize the gamma conversion and multiple scattering, the materials in the acceptance of the detector are kept small in radiation length. The radiation lengths of the windows, mirror panels, and mirror supports are 0.2%, 0.4%, and 1.0%, respectively. The total radiation length of the RICH detector "lled with ethane gas is about 2.1%. A RICH detector has two 16;80 arrays of Hamamatsu H3171S [2] PMTs with UV transparent glass window for Cherenkov photon detection. A PMT has a gain greater than 10 with dark current less than 100 nA. The diameter of the photocathode is 25 mm and its quantum e$ciency is about 20% (5%) at 300 nm (200 nm). Since there is a signi"cant stray magnetic "eld from the central magnet at the place of PMTs, each PMT is housed in a magnetic shielding case. For the "rst 900 PMTs, the shielding case is made of soft iron and mu-metal. For the rest (4220#spares), it is made of ferroperm [3], which has a better shielding capability. A Winstone cone with 50 mm entrance diameter is attached to each PMT to collect Cherenkov light. There are 5120 ("2 (arm);2 (side) ;16 (h) ;80 ( )) PMTs in the RICH system. They are placed near the focal plane of the mirror so that Cherenkov photons with the same emission angles in h and are detected by the same PMT.
The angular segmentation of the PMT array is approximately 13 by 13 in h and . The PMTs are "rst assembled into a 16;2 subarray, called supermodule. After a supermodule is assembled, it is tested and the gain of all 32 tubes are measured. The gain of a tube is determined from the single photo-electron peak. After the gain measurement, the supermodule is made to `burnina at the nominal operating voltage for 3 weeks in a dark room. If any tube fails during the `burn-ina, it is replaced. (The failure rate has been less than 0.5%.) After the `burn-ina, the gain of the tubes are measured again, and then installed in the RICH gas vessel. Forty supermodules forms one 16;80 array of PMTs of one side of the RICH detector, which is shown in Fig. 3. There are two mirror arrays in a RICH detector. One array consists of 2;12 spherically curved graphite-epoxy mirror panels, with total surface of 10 m. The radius of the re#ecting surface of the mirror is 4.03 m. The substrate of a mirror panel consists of 1.25 cm of Rohacell foam sandwiched between two 0.7 mm thick graphite epoxy sheets. The substrate is manufactured by ARDCO [4]. The re#ective surface is added to the substrate by replication, in which an aluminum layer is evaporated onto a highly polished glass master and then
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Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
Fig. 3. One of the PMT arrays of the "rst PHENIX RICH detector. There is an identical PMT array in the opposite side of the RICH.
transferred to the graphite}epoxy substrate. This is accomplished by spacing the substrate 150 lm away from the glass matter, "lling the gap with epoxy, curing the epoxy, and then separating the aluminum layer from the glass master. The replication is done by OPTICON [5]. The re#ectivity of the mirror has been measured from a small sample produced by the same process. The re#ectivity was found 83% at 200 nm and 90% at 250 nm. The mirror panels are supported by a system of graphite}epoxy beams. A mirror panel is mounted to the support beams by three adjustable attachments so that the orientation of the mirror panel can be precisely adjusted. The alignment of the mirror panels were done in the following way. After all mirror panels and PMTs were installed, the
RICH vessel was rotated up in the same orientation as on the PHENIX detector carriage. Then several optical targets were attached on the surface of each of the mirror panels. Their positions were then measured by a computerized theodolite system (MANCAT). The mirror mounts were adjusted so that all optical targets are within 0.25 mm of the designed spherical surface. A test beam experiment with a small prototype RICH was performed at KEK PS and BNL AGS. In the test, the "gure of merit, N , of the device was determined to be about 120 per cm, and its timing resolution for a ring was about 250 ps r.m.s. Pion rejection of better than 10 with electron e$ciency better than 99% for an isolated track was obtained in the test.
Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
147
Fig. 4. Expected performance of the PHENIX RICH in central Au#Au collisions at RHIC. Pion rejection factor vs. electron e$ciency are plotted for ethane, CO , and freon 13. The simulation includes a shielding against background electrons that improves e/p separation by a factor 2.
For the very high particle density expected in heavy ion collisions at RHIC, the pion rejection factor is reduced due to ambiguities in association of a Cherenkov ring with a charged particle track. The photon conversions in the PHENIX detector produces background electrons into RICH active volume. Those electrons produce background hits in RICH PMT array. The background hits and high particle density reduces the e/p separation power of the RICH in Au#Au collisions. This e!ect has been studied by a detailed, GEANT3 based Monte Carlo simulation. Fig. 4 shows estimates of pion rejection factor as a function of electron e$ciency in central Au#Au collisions. Pion rejection of factor 200}1500 with electron e$ciency of 90% to 75% are expected.
The PHENIX RICH detector is in its "nal stage of construction. The assembly of the "rst RICH detector has been completed in October 1998. The entire RICH subsystem will be ready for its "rst operation in 1999. We gratefully acknowlege K. Jones, R. Hoade, and R. Raynis for their technical support. The PHENIX RICH detector is supported by Japanese Ministry of Education, Science, Sports, and Culture, and U.S. Department of Energy.
References [1] PHENIX Conceptual Design Report, BNL, USA 1993, unpublished.
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[2] Hamamatsu Photonics K.K., Hamamatsu 430, Japan. [3] T. Omori et al., J. Appl. Phys 69 (1991) 5927. [4] Advance Ratio Design Co. Inc, 2540 Green Street, Chester, PA 19013, USA.
[5] OPTICON Corporation, 76 Treble Cove Rd, N. Billerica, MA 01862, USA. [6] PHENIX collaboration, Nucl. Phys. A638 (1998) 565c.
Ring imaging Cherenkov detector of PHENIX experiment at RHIC Y. Akiba *, R. Begay, J. Burward-Hoy, R. Chappell, D. Crook, K. Ebisu, M.S. Emery, J. Ferriera, A.D. Frawley, H. Hamagaki, H. Hara, R.S. Hayano, T.K. Hemmick, M. Hibino, R. Hutter, M. Kennedy, J. Kikuchi, T. Matsumoto, C.G. Moscone, Y. Nagasaka, S. Nishimura, K. Oyama, T. Sakaguchi, S. Salomone, K. Shigaki , Y. Tanaka, J.W. Walker, A.L. Wintenberg, G.R. Young High Energy Accelerator Research Organization, Ibaraki 305-0801, Japan State University of New York - Stony Brook, Stony Brook, NY 11794, USA Florida State University, Tallahassee, FL 32306, USA Nagasaki Institute of Applied Science, Nagasaki 851-0193, Japan Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Center for Nuclear Study, University of Tokyo, Tokyo, 188-0002 Japan University of Tokyo, Tokyo 113-0033, Japan Waseda University, Tokyo 169-8555, Japan
Abstract The RICH detector of the PHENIX experiment at RHIC is currently under construction. Its main function is to identity electron tracks in a very high particle density, about 1000 charged particles per unit rapidity, expected in the most violent collisions at RHIC. The design and construction status of the detector and its expected performance are described. 1999 Elsevier Science B.V. All rights reserved.
The PHENIX experiment [1,6] at the Relativistic Heavy Ion Collider (RHIC) is currently under construction. When completed in 1999, RHIC will provide collisions of Au#Au ions at (s"200 GeV per nucleon. The primary goal of PHENIX is to detect a new state of matter, the quark gluon plasma, in heavy ion collisions. The PHENIX detector consists of 4 spectrometers, or
* Corresponding author. E-mail address: [email protected] (Y. Akiba)
`armsa and an inner detector. Two central arms are to measure charged hardons, photons and electrons at the mid-rapidity. They are placed in back-toback fashion on both sides of the beam line, covering 70}1103 degrees in polar angle (h) and 903 per arm in azimuth ( ). The other two arms are to detect muons in forward and backward angles. The inner detector is to measure the total multiplicity, the vertex position, and the start timing of an event. The PHENIX RICH is the primary device for electron identi"cation in the PHENIX central
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 3 1 9 - 8
SECTION III
144
Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
arms. It is a threshold gas Cherenkov detector with a high angular segmentation to cope with the high particle density, about 1000 charged particles per unit rapidity, expected in the most violent collisions at RHIC. Its Cherenkov threshold for pions is 3.5 GeV/c (with ethane radiator) or 4.9 GeV/c (with CO radiator), and thus most pions and heavier particles produce no signal in the RICH. Fig. 1 shows a side view of the RICH detector, and illustrates its function. When an electron passes through the RICH gas volume, Cherenkov photons emitted from the electron are re#ected by thin spherical mirrors and are focused on arrays of PMTs, forming a ring shaped pattern. The direction of the electron track is then measured from the position of the Cherenkov ring. A charged particle track, measured by tracking system of the central arm, is identi"ed as an electron if its direction matches the direction measured from the ring. There are two identical RICH detectors, one for each of the two central arms. The detector is sandwiched by the inner and outer tracking stations of
the PHENIX central arm, and occupies the radial region between r"2.575 and 4.1 m from the beam line. Fig. 2 shows a cut away view of one of the RICH detector. The gas vessel is fabricated from aluminum, and has a volume of approximately 40 m. The vessel houses two arrays of spherical mirrors and two arrays of photo-multiplier tubes (PMTs). The PMT arrays are placed behind the PHENIX central magnet so that particles from the collision vertex do not directly hit the arrays. The vessel is "lled with ethane or CO at 1 atm as Cherenkov radiator. The radiator length, from the entrance window to the surface of the mirror, varies from about 0.9 m at h"903 to 1.5 m at h"703 and 1103. The entrance and exit windows (9 and 21 m) of the vessel are made of 125 lm thick aluminized Kapton. The windows are covered by a light absorber sheet and are supported against a small pressure of radiator gas (1/2 of water) by graphite epoxy rods. All the gas seals are made using GoreTex gaskets.
Fig. 1. A cut through view of PHENIX RICH detector.
Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
145
Fig. 2. Cutaway view of one arm of the PHENIX RICH detector.
In order to minimize the gamma conversion and multiple scattering, the materials in the acceptance of the detector are kept small in radiation length. The radiation lengths of the windows, mirror panels, and mirror supports are 0.2%, 0.4%, and 1.0%, respectively. The total radiation length of the RICH detector "lled with ethane gas is about 2.1%. A RICH detector has two 16;80 arrays of Hamamatsu H3171S [2] PMTs with UV transparent glass window for Cherenkov photon detection. A PMT has a gain greater than 10 with dark current less than 100 nA. The diameter of the photocathode is 25 mm and its quantum e$ciency is about 20% (5%) at 300 nm (200 nm). Since there is a signi"cant stray magnetic "eld from the central magnet at the place of PMTs, each PMT is housed in a magnetic shielding case. For the "rst 900 PMTs, the shielding case is made of soft iron and mu-metal. For the rest (4220#spares), it is made of ferroperm [3], which has a better shielding capability. A Winstone cone with 50 mm entrance diameter is attached to each PMT to collect Cherenkov light. There are 5120 ("2 (arm);2 (side) ;16 (h) ;80 ( )) PMTs in the RICH system. They are placed near the focal plane of the mirror so that Cherenkov photons with the same emission angles in h and are detected by the same PMT.
The angular segmentation of the PMT array is approximately 13 by 13 in h and . The PMTs are "rst assembled into a 16;2 subarray, called supermodule. After a supermodule is assembled, it is tested and the gain of all 32 tubes are measured. The gain of a tube is determined from the single photo-electron peak. After the gain measurement, the supermodule is made to `burnina at the nominal operating voltage for 3 weeks in a dark room. If any tube fails during the `burn-ina, it is replaced. (The failure rate has been less than 0.5%.) After the `burn-ina, the gain of the tubes are measured again, and then installed in the RICH gas vessel. Forty supermodules forms one 16;80 array of PMTs of one side of the RICH detector, which is shown in Fig. 3. There are two mirror arrays in a RICH detector. One array consists of 2;12 spherically curved graphite-epoxy mirror panels, with total surface of 10 m. The radius of the re#ecting surface of the mirror is 4.03 m. The substrate of a mirror panel consists of 1.25 cm of Rohacell foam sandwiched between two 0.7 mm thick graphite epoxy sheets. The substrate is manufactured by ARDCO [4]. The re#ective surface is added to the substrate by replication, in which an aluminum layer is evaporated onto a highly polished glass master and then
SECTION III
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Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
Fig. 3. One of the PMT arrays of the "rst PHENIX RICH detector. There is an identical PMT array in the opposite side of the RICH.
transferred to the graphite}epoxy substrate. This is accomplished by spacing the substrate 150 lm away from the glass matter, "lling the gap with epoxy, curing the epoxy, and then separating the aluminum layer from the glass master. The replication is done by OPTICON [5]. The re#ectivity of the mirror has been measured from a small sample produced by the same process. The re#ectivity was found 83% at 200 nm and 90% at 250 nm. The mirror panels are supported by a system of graphite}epoxy beams. A mirror panel is mounted to the support beams by three adjustable attachments so that the orientation of the mirror panel can be precisely adjusted. The alignment of the mirror panels were done in the following way. After all mirror panels and PMTs were installed, the
RICH vessel was rotated up in the same orientation as on the PHENIX detector carriage. Then several optical targets were attached on the surface of each of the mirror panels. Their positions were then measured by a computerized theodolite system (MANCAT). The mirror mounts were adjusted so that all optical targets are within 0.25 mm of the designed spherical surface. A test beam experiment with a small prototype RICH was performed at KEK PS and BNL AGS. In the test, the "gure of merit, N , of the device was determined to be about 120 per cm, and its timing resolution for a ring was about 250 ps r.m.s. Pion rejection of better than 10 with electron e$ciency better than 99% for an isolated track was obtained in the test.
Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
147
Fig. 4. Expected performance of the PHENIX RICH in central Au#Au collisions at RHIC. Pion rejection factor vs. electron e$ciency are plotted for ethane, CO , and freon 13. The simulation includes a shielding against background electrons that improves e/p separation by a factor 2.
For the very high particle density expected in heavy ion collisions at RHIC, the pion rejection factor is reduced due to ambiguities in association of a Cherenkov ring with a charged particle track. The photon conversions in the PHENIX detector produces background electrons into RICH active volume. Those electrons produce background hits in RICH PMT array. The background hits and high particle density reduces the e/p separation power of the RICH in Au#Au collisions. This e!ect has been studied by a detailed, GEANT3 based Monte Carlo simulation. Fig. 4 shows estimates of pion rejection factor as a function of electron e$ciency in central Au#Au collisions. Pion rejection of factor 200}1500 with electron e$ciency of 90% to 75% are expected.
The PHENIX RICH detector is in its "nal stage of construction. The assembly of the "rst RICH detector has been completed in October 1998. The entire RICH subsystem will be ready for its "rst operation in 1999. We gratefully acknowlege K. Jones, R. Hoade, and R. Raynis for their technical support. The PHENIX RICH detector is supported by Japanese Ministry of Education, Science, Sports, and Culture, and U.S. Department of Energy.
References [1] PHENIX Conceptual Design Report, BNL, USA 1993, unpublished.
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Y. Akiba et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 143}148
[2] Hamamatsu Photonics K.K., Hamamatsu 430, Japan. [3] T. Omori et al., J. Appl. Phys 69 (1991) 5927. [4] Advance Ratio Design Co. Inc, 2540 Green Street, Chester, PA 19013, USA.
[5] OPTICON Corporation, 76 Treble Cove Rd, N. Billerica, MA 01862, USA. [6] PHENIX collaboration, Nucl. Phys. A638 (1998) 565c.